In order to allow a semiconductor device to be high in breakdown voltage and low in loss, silicon carbide has recently increasingly been adopted as a material forming a semiconductor device. Silicon carbide is a wide band gap semiconductor greater in band gap than silicon which has conventionally widely been used as a material forming a semiconductor device. Therefore, by adopting silicon carbide as a material forming a semiconductor device, a higher breakdown voltage and lowering in ON resistance of a semiconductor device can be achieved. A semiconductor device containing silicon carbide as a material is also advantageous in that lowering in characteristics when it is used in a high temperature environment is less than in a semiconductor device containing silicon as a material.
Examples of a semiconductor device containing silicon carbide as a constituent material include a metal oxide semiconductor field effect transistor (MOSFET) and a Schottky barrier diode (SBD). A MOSFET is a semiconductor device in which a current is allowed or not allowed to pass by controlling whether or not an inversion layer is formed in a channel region with a prescribed threshold voltage being defined as a boundary. This MOSFET is disadvantageous in that a stable operation thereof is not obtained due to variation in threshold voltage, across which a current is allowed or not allowed to pass. In order to address this, for example, reduction of instability in threshold voltage has been proposed by studying a condition for forming a gate oxide film (see, for example, Mitsuo Okamoto et al., “Reduction of Instability in Vth of 4H—SiC C-Face MOSFETs,” the 59th Spring Meeting, Proceedings, The Japan Society of Applied Physics, March 2012, pp. 15-309 (NPD 1)).